1. Field of the invention
The present invention relates to an organic electroluminescent display device, more particularly, to an organic electroluminescent display device capable of compensating voltage drop of power supply voltage lines by asymmetrically forming a plurality of contact holes for connecting a cathode electrode and a cathode power supply line.
2. Description of Related Art
FIG. 1 is a plane figure for showing a conventional organic electroluminescent display device.
As shown in FIG. 1, an organic electroluminescent display device 100 may include a pixel region 160 equipped with a plurality of pixels and an upper power supply voltage line 110 arranged at an upper side and left and right sides of the pixel region 160 for power supply voltage. It may also include a lower power supply voltage line 130 arranged at a lower side of the pixel region 160 for power supply voltage and a pixel power supply voltage line 111 arranged correspondingly to the pixel region 160 to connect the upper power supply voltage line 110 and the lower power supply voltage line 130. It may further include a scan driver 140 for outputting selection signals and a data driver 150 for outputting data signals.
Furthermore, the organic electroluminescent display device 100 may additionally include a cathode electrode 122 arranged correspondingly to the pixel region 160, and a cathode power supply line 120 (not shown in FIG. 1) formed at one side of the pixel region 160. The cathode power supply line 120 may be equipped with contact holes for connecting the cathode power supply line 120 to the cathode electrode 122.
FIG. 2 illustrates a plane structure of a cathode power supply line equipped with one contact hole for connecting the cathode power supply line to the cathode electrode in a conventional organic electroluminescent display device.
As shown in FIG. 2, one contact hole 121 may be arranged on a cathode power supply line 120 so that the cathode power supply line 120 is connected to a cathode electrode 122 through the contact hole 121. A cathode voltage supplied to the cathode power supply line 120 from an external terminal may be supplied to the cathode electrode 122 through the contact hole 121.
In a conventional organic electroluminescent display device having the foregoing structure, if selection signals and data signals are provided to the pixel region 160 from the scan driver 140 and data driver 150, power supply voltage may be provided to the pixel power supply voltage line 111 from the power supply voltage lines 110 and 130. The cathode voltage may be provided to the cathode electrode 122 from the cathode power supply line 120. Switching transistor and driving transistor (not shown in FIG. 2) composing pixels arranged on the pixel region 160 may be driven so that electroluminescent elements (not shown in FIG. 2) emit lights.
FIG. 3 illustrates conventional distribution of power supply voltage supplied to the pixel region 160 from the power supply voltage lines 110 and 130 in an organic electroluminescent display device illustrated on FIG. 1.
As shown in FIG. 3, the distribution of the power supply voltage in the pixel region 160 may be divided into a region to which a relatively lower power supply voltage is provided, and one in which a relatively higher power supply voltage is provided. These may be selected based on the distance from the power supplying component, because the farther away the pixel region 160 is from a power supply voltage supplying component, the greater the voltage drop is. This voltage drop may be attributed to resistance loss (IR loss) in the line. Furthermore, the same effect can be observed in the cathode power supply lines.
That is, a relatively higher power supply voltage may be supplied to a part adjacent to the power supply voltage supplying side of the circuit, than to the remainder of the line. Similarly, a relatively higher cathode voltage may be supplied to a part adjacent to an external terminal, than to the more distant parts.
There have been conventional problems with non-uniformity of luminance in the pixel region. These problems may be further worsened because the part that experiences the voltage drop of the cathode electrode may also be the part having a high voltage drop of power supply voltage in the pixel region. Thus the two regions may overlap. Furthermore, there also have been conventional problems that emission luminance of the pixel region may become even less uniform because the cathode power supply line may be only at one side of the pixel region.